1. Field of the Invention
The present invention relates to a light source unit suitable for use as a light source in a liquid crystal display apparatus or the like, relates also to an illumination apparatus incorporating such a light source unit and designed for use in a display apparatus, and relates further to a display apparatus incorporating such an illumination apparatus designed for use in a display apparatus.
2. Description of the Related Art
In liquid crystal display apparatuses widely used as display apparatuses, to achieve bright, easily readable display, a backlight is disposed behind a transmissive liquid crystal display panel. The backlight is typically built with a plurality of cold-cathode tubes. Each cold-cathode tube is composed of a glass tube filled with a discharge gas, and has discharge electrodes provided one inside each end portion of the glass tube. An intermittent electric current is fed from an inverter circuit, serving as a control circuit for controlling the cold-cathode tube, to the discharge electrodes to excite the discharge gas inside the glass tube, which causes the cold-cathode tube to light.
The cold-cathode tube has a characteristic (negative characteristic) such that, as the current fed thereto increases, the discharge gas inside the glass tube is excited such that the total number of electrons therein increases. As a result, the resistance across the cold-cathode tube decreases. Thus, when the cold-cathode tube is energized by being fed with a constant voltage from the control circuit, as the resistance across the cold-cathode tube decreases, the current further increases. An excessive increase in the current that flows through the cold-cathode tube may destroy it.
To overcome this problem, a method for preventing a cold-cathode tube from being fed with an excessive current has been used. FIG. 10 is a diagram schematically showing the configuration of a control circuit that is used to practice such a method. A cold-cathode tube 10a is composed of a glass tube 10b filled with a discharge gas, with discharge electrodes 10c provided one inside each end portion of the glass tube 10b. The discharge gas fills the interior of the glass tube 10b. The discharge electrodes 10c are connected to a connector 44 provided in a control circuit 40 configured as an inverter circuit.
The control circuit 40 includes a low-voltage alternating current source 41 and a step-up transformer 42 that steps up the voltage of the low-voltage alternating current outputted from the low-voltage alternating current source 41. The step-up transformer 42 produces an intermittent electric current, which is then fed via a current-limiting capacitor 43, serving as a ballast capacitor, to the connector 44, and is then fed from the connector 44 to the discharge electrodes 10c of the cold-cathode tube 10a. The control circuit 40 further includes a current feedback circuit 45 that controls, according to the current that flows through the cold-cathode tube 10a, the current fed from the current-limiting capacitor 43 to the cold-cathode tube 10a so that this current remains constant.
In the control circuit 40, the alternating-current voltage from the low-voltage alternating current source 41 is stepped up by the step-up transformer 42, and is then fed via the current-limiting capacitor 43 to the discharge electrodes 10c of the cold-cathode tube 10a. With this configuration, the current-limiting capacitor 43, by controlling the intermittent voltage outputted from the step-up transformer 42, controls the current fed to the cold-cathode tube 10a such that the current that flows through the cold-cathode tube 10a does not increase even as the resistance across it decreases.
On the other hand, Japanese Patent Application Laid-open No. H6-222721 discloses a configuration in which the capacitances of current-limiting capacitors are made variable for the purpose of making uniform the brightness of the cold-cathode tubes constituting a backlight.
In the configuration shown in FIG. 10, where the control circuit 40 includes the current-limiting capacitor 43, the voltage from the step-up transformer 42 is applied in a form divided between across the current-limiting capacitor 43 and across the cold-cathode tube 10a. Thus, when a predetermined voltage that needs to be applied across the cold-cathode tube 10a to make it start to light, the step-up transformer 42 needs to output an excessively high voltage. This greatly degrades the reliability of the control circuit 40 against high voltages, and makes it difficult to use long cold-cathode tubes that are designed for use in backlights for large-screen liquid crystal display panels, which inherently require high voltages. It is also necessary to greatly increase the size of the step-up transformer 42. This increases the footprint of the control circuit 40 on the circuit board, and leads to poor economy.
Moreover, the voltage obtained by the action of the current-limiting capacitor 43 greatly varies with the characteristics of the current-limiting capacitor 43, environmental conditions, and other factors, and this makes it difficult to feed a constant current to the cold-cathode tube 10a. For this reason, the control circuit includes the current feedback circuit 45, which feeds a constant current to the cold-cathode tube 10a. The provision of this current feedback circuit 45, however, contributes to the increased footprint on the circuit board and to poor economy. Moreover, where a plurality of cold-cathode tubes 50a are provided, a plurality of current feedback circuits 45 need to be provided one for each of them. This greatly increases the number of expensive feedback circuits needed, leading to poor economy and an increased footprint on the circuit board.
By contrast, when the control circuit 40 includes, for each of a plurality of cold-cathode tubes 10a, one step-up transformer 42, one current-limiting capacitor 43, and one current feedback circuit 45, part of the cold-cathode tubes 10a may fail to light, or, among the cold-cathode tubes 10a that have managed to light, the currents fed thereto may vary, resulting in uneven brightness among the cold-cathode tubes 10a. The lifetimes of the cold-cathode tubes 10a may also vary greatly. A backlight built with a plurality of cold-cathode tubes in this way cannot uniformly illuminate the entire display surface of a liquid crystal display panel, leading to poor display quality. This inconvenience is experienced not only in liquid crystal display apparatuses but also in display apparatuses of any other type in which a display panel is illuminated with a backlight.